The five ultrasound features of thyroid nodules used in TI-RADS are: composition, echogenicity, shape, margin and punctate echogenic foci. Each item is given points.

The points are added from all categories to determine the TI-RADS level, each with a recommendation.

Nodules smaller than 5 mm do not need any follow-up, even if they are TI-RADS 5.
This is because it is very unlikely that nodules smaller than 5 mm will become a clinical significant malignancy.

The cutoff point of 2.5 cm for fine needle aspiration (FNA) in mildly suspicious TR3 lesions is based on studies showing that thyroid carcinomas don’t have a decreased survival until they reach this threshold value.

The ACR-TIRADS category has been shown to have good correlation with the malignancy risk in large studies.

The risk of malignancy is:

TR1: 0.3%
TR2: 1.5%
TR3: 4.8%
TR4: 9.1%
TR5: 35%

There are several exceptions for TI-RADS, where this system cannot be used for.

Each of these exceptions have a greater risk of thyroid cancer than the otherwise normal adult population:

Children
FDG-PET avid thyroid nodules
Lymphadenopathy
Known risk factors for thyroid malignancy, like MEN type 2

FNA biopsy is recommended for suspicious lesions (TR3-TR5) with the above size criteria. If there are multiple nodules, the two with the highest ACR TI-RADS scores should be sampled (rather than the two largest), with largest size being used a tie-breaker if there are multiple nodules of the same classification.

Interval enlargement on follow-up is significant if there is an increase of >20% and >2 mm in two dimensions or a >50% increase in volume. If the ACR TI-RADS level increases between scans, an interval scan the following year is again recommended.